Aşağı-çevirici polimer kompozitler ve bunların beyaz ışık uygulamaları

Thesis (Doctoral)--Izmir Institute of Technology, Materials Science and Engineering, Izmir, 2018Includes bibliographical references (leaves. 75-94)Text in English; Abstract: Turkish and English.Down-converting materials, which are luminescent materials that can emit at lower energies than their excitation energies, have been employed in a broad range of application area including phosphor-converted white LEDs, display technologies, solar cells, etc. In general, they can be obtained in the form powder or in dispersion. Therefore, to use them in an application, various methods such as spin coating can be applied or they can be prepared in the form of ceramic plate or polymer composite. In this thesis, we fabricate polymeric composite of some of these down-converting materials to use them in white light applications. In this sense, we have prepared these down-converting materials together with suitable polymers to form polymer/down-converting material composite in order to obtain free-standing film. Among those, polydimethylsiloxane was used in most of our applications especially the ones that involve inorganic downconverting materials. Second, polystyrene was employed to produce electrospun fibers in the case of when organic down-converting materials were used. After obtaining these composites in the form of free-standing film, we have focused specifically their use as color conversion layers over blue or UV LED chip to produce white light. The last part of the thesis describes our future prospects. We think that halide perovskites, which have high quantum yield, low cost, ease of synthesis, and wavelength tunability, can be promising materials in order to be employed as color conversion layer in white light applications. The details of these material systems, and their recent use in phosphorconverted white LED applications were summarized.Aşağı-çevirici malzemeler, uyarıldıkları enerjiden daha düşük enerjilerde ışıma yapan ışıldayan malzemelerdir, fosfor-dönüşümlü beyaz LED, ekran teknolojileri, güneş hücreleri gibi geniş bir alanda kullanılmaktadırlar. Bunlar genel olarak toz veya ıslak olarak elde edilirler. Bu sebeple, bir uygulamada kullanmak için döndürmeli kaplama gibi yöntemler uygulanmalı veya seramik plaka ve polimer kompozit şeklinde hazırlanmalıdırlar. Bu tezde, beyaz ışık uygulamaları için kullanmak üzere bazı aşağıçevirici malzemelerin polimer kompozitlerini ürettik. Bu manada, kendi başına ayakta durabilen film elde etmek amacıyla bu aşağı-çevirici malzemeler uygun polimerler ile birlikte polimer/aşağı-çevirici malzeme kompozit olarak hazırladık. Bunlar içerisinden, ilk olarak, çoğu uygulamamızda özellikle inorganik aşağı-çevirici malzeme içerenlerde polydimethylsiloxane kullandık. İkinci olarak, organik aşağı-çevirici malzemelerin kullanıldığı durumlarda elektrospun lifleri üretmek için polistiren kullanılmıştır. Bu kendi başına ayakta duran kompozitleri elde ettikten sonra beyaz ışık üretmek için spesifik olarak bunların mavi ya da UV LED üzerinde renk dönüşüm katmanları olarak kullanımına odaklandık. Tezin son kısmı gelecek çalışma olasılıklarını betimlemektedir. Yüksek kuantum verimliliklerinden, düşük maliyetlerinden, kolay sentezlenmelerinden ve dalgaboyu ayarlanabilirliklerinden ötürü halojenür perovskite’ların beyaz ışık uygulamalarında renk dönüşüm katmanı olarak kullanılmalarının umut vaad ettiğini düşünüyoruz. Bu malzeme sistemlerinin detayları ve bunların son dönemlerde fosfordönüşümlü beyaz LED uygulamalarındaki kullanımları özetlendi

Entropik tünelleme zamanı ve uygulamaları

Thesis (Master)--Izmir Institute of Technology, Physics, Izmir, 2014Includes bibliographical references (leaves: 56-61)Text in English; Abstract: Turkish and Englishviii, 75 leavesQuantum tunneling is one of the most interesting consequences of quantum behaviour. However, even though it is known and understood well, due to non-existence of a time operator in quantum mechanics, estimating what time it takes for a particle to cross a barrier remains an open question. There are some attempts like phase time, dwell time, Larmor clock, Buttiker-Landauer, and Feynman Path Integral approaches. These definitions do not agree with each other and with experiment. In this thesis work, tunneling time problem is studied in a rather new context. Knowing time necessitates momentum, we deal with momentum state vectors and define entropy accordingly. This entropy, which gives a temperature, defines a thermal energy in the tunneling region. With this thermal energy and uncertainty principle, resulting time deviation of the particle from the classically instantaneous is stated as our tunneling time, entropic tunneling time. Moreover, we compare this tunneling time with recent experiments in detail, and find that, it is in very good agreement with the data. Then, we apply this entropic tunneling time to α - decay, STM and creation of universe from nothing to predict natural time scale of these processes

Aşağı-çevirici polimer kompozitler ve bunların beyaz ışık uygulamaları

Thesis (Doctoral)--Izmir Institute of Technology, Materials Science and Engineering, Izmir, 2018Includes bibliographical references (leaves. 75-94)Text in English; Abstract: Turkish and English.Down-converting materials, which are luminescent materials that can emit at lower energies than their excitation energies, have been employed in a broad range of application area including phosphor-converted white LEDs, display technologies, solar cells, etc. In general, they can be obtained in the form powder or in dispersion. Therefore, to use them in an application, various methods such as spin coating can be applied or they can be prepared in the form of ceramic plate or polymer composite. In this thesis, we fabricate polymeric composite of some of these down-converting materials to use them in white light applications. In this sense, we have prepared these down-converting materials together with suitable polymers to form polymer/down-converting material composite in order to obtain free-standing film. Among those, polydimethylsiloxane was used in most of our applications especially the ones that involve inorganic downconverting materials. Second, polystyrene was employed to produce electrospun fibers in the case of when organic down-converting materials were used. After obtaining these composites in the form of free-standing film, we have focused specifically their use as color conversion layers over blue or UV LED chip to produce white light. The last part of the thesis describes our future prospects. We think that halide perovskites, which have high quantum yield, low cost, ease of synthesis, and wavelength tunability, can be promising materials in order to be employed as color conversion layer in white light applications. The details of these material systems, and their recent use in phosphorconverted white LED applications were summarized.Aşağı-çevirici malzemeler, uyarıldıkları enerjiden daha düşük enerjilerde ışıma yapan ışıldayan malzemelerdir, fosfor-dönüşümlü beyaz LED, ekran teknolojileri, güneş hücreleri gibi geniş bir alanda kullanılmaktadırlar. Bunlar genel olarak toz veya ıslak olarak elde edilirler. Bu sebeple, bir uygulamada kullanmak için döndürmeli kaplama gibi yöntemler uygulanmalı veya seramik plaka ve polimer kompozit şeklinde hazırlanmalıdırlar. Bu tezde, beyaz ışık uygulamaları için kullanmak üzere bazı aşağıçevirici malzemelerin polimer kompozitlerini ürettik. Bu manada, kendi başına ayakta durabilen film elde etmek amacıyla bu aşağı-çevirici malzemeler uygun polimerler ile birlikte polimer/aşağı-çevirici malzeme kompozit olarak hazırladık. Bunlar içerisinden, ilk olarak, çoğu uygulamamızda özellikle inorganik aşağı-çevirici malzeme içerenlerde polydimethylsiloxane kullandık. İkinci olarak, organik aşağı-çevirici malzemelerin kullanıldığı durumlarda elektrospun lifleri üretmek için polistiren kullanılmıştır. Bu kendi başına ayakta duran kompozitleri elde ettikten sonra beyaz ışık üretmek için spesifik olarak bunların mavi ya da UV LED üzerinde renk dönüşüm katmanları olarak kullanımına odaklandık. Tezin son kısmı gelecek çalışma olasılıklarını betimlemektedir. Yüksek kuantum verimliliklerinden, düşük maliyetlerinden, kolay sentezlenmelerinden ve dalgaboyu ayarlanabilirliklerinden ötürü halojenür perovskite’ların beyaz ışık uygulamalarında renk dönüşüm katmanı olarak kullanılmalarının umut vaad ettiğini düşünüyoruz. Bu malzeme sistemlerinin detayları ve bunların son dönemlerde fosfordönüşümlü beyaz LED uygulamalarındaki kullanımları özetlendi

Statistical approach to tunneling time in attosecond experiments

Tunneling, transport of particles through classically forbidden regions, is a pure quantum phenomenon. It governs numerous phenomena ranging from single-molecule electronics to donor–acceptor transition reactions. The main problem is the absence of a universal method to compute tunneling time. This problem has been attacked in various ways in the literature. Here, in the present work, we show that a statistical approach to the problem, motivated by the imaginary nature of time in the forbidden regions, lead to a novel tunneling time formula which is real and subluminal (in contrast to various known time definitions implying superluminal tunneling). In addition to this, we show explicitly that the entropic time formula is in good agreement with the tunneling time measurements in laser-driven He ionization. Moreover, it sets an accurate range for long-range electron transfer reactions. The entropic time formula is general enough to extend to the photon and phonon tunneling phenomena

A review on halide perovskites as color conversion layers in white light emitting diode applications

In the last decade, halide perovskites have attracted great interest due to many reasons, including their low cost, solution-processability, superior PL properties, broad range of color tunability, color purity, and defect tolerance, among others. They are in increasing demand for a wide range of applications, such as solar cells, light emitting diodes (LEDs), white LEDs (WLED), and lasers. Yellow phosphors have been heavily employed in solid-state lighting, since its illumination by blue yields white light with various optical features, such as high/low CRI, CCT, and luminous efficiency. However, phosphors as pigments are seldom found and are costly material. In this sense, halide perovskites may offer a promising alternative to phosphors for future solid-state lighting. In this study, the synthesis and optical properties of halide perovskites, as well as their application in WLED as a conversion layer are investigated.Turkish Academy of Science

A climate impact of dam lake and Recep Yazicioglu Gokpinar dam lake sample

<span style="color: #515151;">Dams have both positive and negative effects on the environment. The most important effect can be defined as the regional climate change. In this study, the impact on Denizli city climate of Vali Recep Yazıcıoğlu Gökpınar Dam Lake was determined. In this study, Denizli city meteorological station (maximum, minimum and mean temperatures, total precipitation and wind speed) data were used. Before and after from the dam construction; absolute changes are examined. After the dam construction; maximum, minimum and average temperature increase was observed in all months. A rainfall was observed to be increased in January, February and October on after dam construction, but another times was observed to be decreased. The evaporation and wind speed parameter has been little change. Linear regression, the Mann-Kendall and Sen’s Method for trend analysis were used. All techniques considered here consistently reveal that there are upward trend in maximum, minimum and mean temperatures, wind speed, no trend in precipitation and evaporation.</span

Non-iridescent structural colors from uniform-sized SiO2 colloids

Structural colors have recently attracted interest from diverse fields of research due to their ease of fabrication and eco-friendliness. These types of colors are, in principle, achieved by periodically arranged submicron-diameter colloidal particles. The interaction of light with a structure containing long-range ordered colloidal particles leads to coloration; this usually varies depending on the angle of observation (iridescence). However, the majority of the applications demand constant color that is independent of the viewing angle (non-iridescence). In this work, silica colloids were obtained using the Stöber method at different sizes from 150 to 300 nm in an alcoholic dispersion. The casting of the dispersion on a substrate leaves behind a photonic crystal showing a colorful iridescent film. However, centrifugation and redispersion of the SiO2 particles into fresh solvent may cause the formation of small, aggregated silica domains in the new dispersion. The casting of this dispersion allows for the development of photonic glass, presumably due to the accumulation of aggregates showing stable colloidal film independent of viewing angle. Moreover, depending on the size of the silica colloids, non-iridescent photonic glasses with various colors (violet, blue, green, and orange) are obtained.Turkish Academy of Science

Multilayer design of hybrid phosphor film for application in LEDs

Crosslinked polydimethylsiloxane (PDMS) composite coatings containing luminescent micrometer-sized yellow Y3Al5O12:Ce3+ (YAG:Ce3+) particles were prepared by spraying for potential applications in solid-state lighting. Blue light was down converted by phosphor particles to produce white light, yet poor color properties of YAG:Ce3+ stemmed from a deficiency of red. When nitride-based red phosphor was simply blended into the system, the electrostatic interaction of negatively charged YAG:Ce3+ and positively charged red phosphor particles caused remarkable clustering and heterogeneity in particle dispersion. Consequently, the light is dominantly blue and shifted to cold white. In other case, phosphor particles were sprayed onto the diffused polycarbonate substrate in stacked layers. Coatings with >80% inorganic content by mass with a thickness of 60 μm were subjected to thermal crosslinking, which the presence of the phosphor particles obstructed, presumably due to the hindrance of large phosphor particles in the diffusion of PDMS precursors. The coating of YAG:Ce3+ first followed by red phosphor in stacked layers produced better light output and color properties than the coating obtained by spraying the mixture at once. Monte Carlo simulation validated the hypothesis.Turkish Academy of Sciences [TUBA-GEBIP]; Scientific and Technological Council of Turkey, TEYDEB (1501 3150192

Optical enhancement of phosphor-converted wLEDs using glass beads

YAG:Ce3+ is a yellow-source compound commonly used in phosphor conversion layers for direct coating or remote phosphor configurations in LED illumination. This material, however, suffers from a high correlated color temperature, and low color-rendering index due to its deficiency in the red spectrum. In this study, glass beads (GB) with an average particle diameter of 10 μm were introduced to the conversion layer of a YAG:Ce3+ particulate-filled polydimethylsiloxane matrix composite structure and found to improve the optical features of the resulting composite.Turkish Academy of Sciences (TUBA-GEBIP

Strength and microstructure evolution in cemented mine backfill with low and high pH pyritic tailings: Effect of mineral admixtures

To properly dispose mining-induced tailings, diverse techniques including cemented paste fill (CPB) have been already employed. Indeed, tailings are manufactured by using different admixtures which are poisonous to the environment and need to be closely explored by considering its effects. This study dealt with the quality of CPB containing low pH (4.9) and high pH (10.8) pyritic tailings. Besides, employing ground blast furnace slag (GBFS) and fly ash (FA) in diverse rates instead of ordinary Portland cement (OPC) and their effects on fill performance were explored. Strength and microstructure of CPB made with constant solid content (72 wt%) and different OPC/GBFS-FA ratios (e. g., 90/10, 70/30 and 50/50) were carried out by several laboratory tests. Results showed that the lowest UCS performance was obtained from CPB having 50%FA and acidic tailings. It was also observed that GBFS-based backfills gave better strength than FA-based ones, irrespective of tailings, owing to the former's high pozzolanic activity and grain shape. Regardless of additives, the highest strengths were gained from basic tailings-based backfills. pH of pyritic tailings affected fill's performance due to its aggressive effects on cement hydration. To sum up, this study has provided relevant information and technical evidence that will subsidize the engineering design of a low-cost, zero-waste, and viable mine fill system